Sheet feeding device and image forming apparatus

ABSTRACT

Provided is a sheet feeding device and an image forming apparatus capable of performing sheet feeding by electrostatic adsorption at a low noise with a simple configuration. An adsorbing member  410  includes a straight line portion  410   m  and a curved portion  410   n , and the adsorbing member  410  adsorbs a sheet through the curved portion  410   n  and conveys the sheet upward through the straight line portion  410   m.

TECHNICAL FIELD

The present invention relates to a sheet feeding device and an image forming apparatus, and more particularly, to a technique of feeding a sheet using electrostatic adsorption force.

BACKGROUND ART

An image forming apparatus such as a copying machine or a printer according to a related art includes a sheet feeding device that feeds a sheet, and as the sheet feeding device, there is a friction feed system in which a topmost sheet is separated and fed from a cassette on which a sheet bundle is loaded using frictional force of a rubber roller or the like. In the sheet feeding device of the friction feed system, the topmost sheet is fed by the rubber roller rotating while pressing the sheet bundle. Here, when a sheet is fed, multi-sheet feeding in which a plurality of sheets are conveyed by friction between sheets may occur. On the other hand, conveyance resistance works on the remaining sheets excluding the topmost sheet through a separating pad or a retard roller, and thus only the topmost sheet is fed to an image forming portion.

Meanwhile, in the sheet feeding device of the friction separation system, since the rubber roller feeds a sheet while applying great pressure to the sheet, noise generated by sliding friction between sheets or between the sheet and the rubber roller is problematic. In addition, when the multi-sheet feeding caused by the separating pad or the retard roller is prevented, sliding fricative between sheets is greatly generated. Further, since the separating pad or the retard roller serves as conveyance resistance of the topmost sheet even when the multi-sheet feeding does not occur, a sound is generated by stick slip between the separating pad or the retard roller and the sheet.

In this regard, as a technique of solving the problem, there is a sheet feeding device configured to separate and feed a sheet while adsorbing the sheet using electrostatic adsorption force, specifically, by an electric field formed on a belt surface (see Patent Literatures 1 and 2). In the sheet feeding device of the electrostatic adsorption separation system, since it is possible to convey the topmost sheet as if the topmost sheet is peeled off from the sheet bundle, it is possible to significantly reduce noise generated in a feeding portion.

CITATION LIST Patent Literature Patent Literature 1: Japanese Patent Laid-Open No. 2011-168396 Patent Literature 2: Japanese Patent Laid-Open No. 5-139548 SUMMARY OF INVENTION Technical Problem

However, in the sheet feeding device of the related art that feeds the sheet using electrostatic adsorption force, in a configuration of Patent Literature 1, it is possible to apply sufficient electrostatic adsorption force to the sheet, but when the sheet is separated, since lifting and lowering are performed for each frame on which the adsorbing belt is carried, an operation sound occurs. A collision sound with the sheet occurs as well. Further, when the sheet is adsorbed, belt tension is reduced by reducing an inter-axial distance so that a sheet can be adsorbed with certainty even when a sheet curls, that is, so that followability to the sheet curl can be secured when the adsorbing belt adsorbs the sheet. However, when the sheet is adsorbed in a state in which belt tension is reduced, it is necessary to increase tension at the time of the separation operation, and when the tension is increased as described above, string vibration occurs in the belt, and a sudden sound is caused by the vibration.

In a configuration of Patent Literature 2, the adsorbing belt is used, but since the sheet separation operation is performed by causing the carrying roller to perform an eccentric motion instead of lifting and lowering the adsorbing belt for each frame, a machinery operation sound is reduced. However, when the adsorbing belt comes into contact with the sheet bundle with certainty, the roller collides with the sheet bundle through the adsorbing belt, and thus a collision sound still occurs. Further, when an attempt to prevent a collision between the roller and the sheet bundle is made, the belt is separated from the sheet bundle, sheet adsorption by the adsorbing belt becomes unstable, leading to a feeding failure.

In this regard, in light of the foregoing, it is an object of the present invention to provide a sheet feeding device and an image forming apparatus, which are capable of stably performing sheet feeding by electrostatic adsorption at a low noise with a simple configuration.

Solution to Problem

The present invention provides a sheet feeding device which includes a loading unit that loads a sheet, a first rotating member that is arranged above the loading unit, an adsorbing member that includes a first portion and a second portion having a curvature larger than a curvature of the first portion and electrically adsorbs the sheet loaded on the loading unit, an inner side of the adsorbing member being supported by the first rotating member, a nip member that nips the adsorbing member together with the first rotating member, a first driving unit that rotates the first rotating member and the first nip member, and a control unit that controls the driving unit, wherein the control unit adsorbs the sheet loaded on the loading unit through the second portion and then conveys the sheet upward through the first portion.

Advantageous Effects of Invention

According to the present invention, the adsorbing member that electrically adsorbs the sheet includes a first portion and a second portion having a curvature larger than a curvature of the first portion, and the adsorbing member adsorbs the sheet loaded on the loading unit through the second portion and then conveys the sheet upward through the first portion. Thus, it is possible to stably perform sheet feeding by electrostatic adsorption at a low noise with a simple configuration.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a schematic configuration of an image forming apparatus equipped with a sheet feeding device according to a first embodiment of the present invention.

FIG. 2 is a diagram for describing a configuration of the sheet feeding device.

FIG. 3 is a control block diagram of the sheet feeding device.

FIG. 4 is a diagram for describing a detailed configuration of an adsorbing member of a sheet adsorption separation feeding portion installed in the sheet feeding device and a generation principle of adsorption force by which the adsorbing member adsorbs a sheet.

FIG. 5 is a diagram for describing the adsorbing member of the sheet adsorption separation feeding portion installed in the sheet feeding device.

FIG. 6 is a diagram for describing a sheet separation feeding operation of the sheet adsorption separation feeding portion.

FIG. 7 is a timing chart of a time of sheet separation feeding of the sheet adsorption separation feeding portion.

FIG. 8 is a diagram for describing a configuration of a sheet feeding device according to a second embodiment of the present invention.

FIG. 9 is a diagram for describing a configuration of an adsorbing member of a sheet adsorption separation feeding portion installed in the sheet feeding device.

FIG. 10 is a diagram for describing a sheet separation feeding operation of the sheet adsorption separation feeding portion.

FIG. 11 is a timing chart of a time of sheet separation feeding of the sheet adsorption separation feeding portion.

FIG. 12 is a diagram for describing a configuration of a sheet feeding device according to a third embodiment of the present invention.

FIG. 13 is a diagram for describing a detailed configuration of an adsorbing member of a sheet adsorption separation feeding portion installed in the sheet feeding device and a generation principle of adsorption force by which the adsorbing member adsorbs a sheet.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present invention will be described in detail with reference to the appended drawings. FIG. 1 is a diagram illustrating a schematic configuration of an image forming apparatus equipped with a sheet feeding device according to a first embodiment of the present invention.

In FIG. 1, 100 indicates an image forming apparatus, and 100A indicates an image forming apparatus body (hereinafter, referred to as an “apparatus body”). An image reading portion 41 that includes an image sensor of irradiating an original placed on a platen glass serving as an original placing platen with light and converting reflected light into a digital signal and the like is arranged above the apparatus body 100A. An original from which an image is read is conveyed on the platen glass by an automatic original feeding device 41 a. An image forming portion 55, sheet feeding devices 51 and 52 of feeding a sheet S to the image forming portion 55, and a sheet reversing portion 59 of reversing the sheet S and conveying the reversed sheet S to the image forming portion 55 are arranged in the apparatus body 100A.

The image forming portion 55 includes an exposure unit 42 and four process cartridges 43 (43 y, 43 m, 43 c, and 43 k) for forming toner images of four colors, that is, yellow (Y), magenta (M), cyan (C), and black (Bk). The image forming portion 55 further includes an intermediate transfer unit 44, a secondary transfer portion 56, and a fixing portion 57 arranged above the process cartridge 43.

Here, the process cartridge 43 includes a photosensitive drum 21 (21 y, 21 m, 21 c, and 21 k), a charging roller 22 (22 y, 22 m, 22 c, and 22 k), and a developing roller 23 (23 y, 23 m, 23 c, and 23 k). The process cartridge 43 further includes a drum cleaning blade 24 (24 y, 24 m, 24 c, and 24 k).

The intermediate transfer unit 44 includes a belt driving roller 26, an intermediate transfer belt 25 stretching to an inner secondary transfer roller 56 a or the like, and primary transfer roller 27 (27 y, 27 m, 27 c, and 27 k) that abuts the intermediate transfer belt 25 at a position opposite to the photosensitive drum 21. As will be described later, as transfer bias of a positive polarity is applied to the intermediate transfer belt 25 through the primary transfer roller 27, toner images having a negative polarity on the photosensitive drum 21 are sequentially multi-transferred onto the intermediate transfer belt 25. As a result, a full color image is formed on the intermediate transfer belt 25.

The secondary transfer portion 56 is configured with the inner secondary transfer roller 56 a and an outer secondary transfer roller 56 b that comes into contact with the inner secondary transfer roller 56 a with the intermediate transfer belt 25 interposed therebetween. Further, as will be described later, as secondary transfer bias of a positive polarity is applied to the outer secondary transfer roller 56 b, the full color image formed on the intermediate transfer belt 25 is transferred onto the sheet S.

The fixing portion 57 includes a fixing roller 57 a and a fixing backup roller 57 b. The sheet S is nipped and conveyed between the fixing roller 57 a and the fixing backup roller 57 b, and thus the toner image on the sheet S is pressed and heated, and then fixed onto the sheet S. The sheet feeding devices 51 and 52 include cassettes 51 a and 52 a, respectively, serving as a storage unit that stores the sheet S and sheet adsorption separation feeding portions 51 b and 52 b, respectively, having a function of feeding the sheets S one by one while adsorbing the sheet S stored in the cassettes 51 a and 52 a by static electricity.

In FIG. 1, 103 indicates a pre-secondary transfer conveyance path in which the sheet S fed from the cassettes 51 a and 52 a is conveyed to the secondary transfer portion 56, and 104 indicates a pre-fixing conveyance path in which the sheet S conveyed to the secondary transfer portion 56 is conveyed from the secondary transfer portion 56 to the fixing portion 57. 105 indicates a post-fixing conveyance path in which the sheet S conveyed to the fixing portion 57 is conveyed from a fixing portion 57 to a switching member 61, and 106 indicates a discharge path in which the sheet S conveyed to the switching member 61 is conveyed from the switching member 61 to a discharge portion 58. 107 is a re-conveyance path in which the sheet S reversed by the sheet reversing portion 59 is conveyed to the image forming portion 55 again in order to form an image on a reverse side of the sheet S having an image formed on one surface thereof by the image forming portion 55.

Next, an image forming operation of the image forming apparatus 100 having the above configuration will be described. When the image forming operation starts, the exposure unit 42 first irradiates the surface of the photosensitive drum 21 with laser beams based on image information provided from a personal computer (not illustrated) or the like. At this time, the surface of the photosensitive drum 21 is uniformly charged to a predetermined polarity and potential by the charging roller 22, and when the laser beams are irradiated, charges of a portion irradiated with the laser beams are attenuated, and thus an electrostatic latent image is formed on the surface of the photosensitive drum.

Thereafter, the electrostatic latent image is developed by yellow (Y), magenta (M), cyan (C), and black (Bk) toners supplied from the developing roller 23, and thus the electrostatic latent image is visualized as toner images. Then, the toner images of the respective colors are sequentially transferred onto the intermediate transfer belt 25 by primary transfer bias applied to the primary transfer roller 27, and thus a full color toner image is formed on the intermediate transfer belt 25.

On the other hand, in parallel with the toner image forming operation, in the sheet feeding devices 51 and 52, only one piece of sheet S is separated and fed from the cassettes 51 a and 52 a through the sheet adsorption separation feeding portions 51 b and 52 b. Thereafter, the sheet S is detected by sheet leading end detecting sensors 51 c and 52 c and reaches a pair of drawing rollers 51 d and 51 e. Further, the sheet S nipped between the pair of drawing rollers 51 d and 51 e is fed to the conveyance path 103 and abuts a pair of registration rollers 62 a and 62 b that is stopped, so that a position of the leading end thereof is adjusted.

Then, in the secondary transfer portion 56, the pair of registration rollers 62 a and 62 b are driven at a timing at which the full color toner image on the intermediate transfer belt matches the position of the sheet S. As a result, the sheet S is conveyed to the secondary transfer portion 56, and in the secondary transfer portion 56, the full color toner image is collectively transferred onto the sheet S through secondary transfer bias applied to the outer secondary transfer roller 56 b.

The sheet S onto which the full color toner image has been transferred is conveyed to the fixing portion 57 and receives heat and pressure in the fixing portion 57, and the toners of the respective colors undergo melting and color mixture and are fixed as a full color image to the sheet S. Thereafter, the sheet S to which the image has been fixed is discharged through the discharge portion 58 installed in the downstream of the fixing portion 57. Further, when an image is formed on both sides of the sheet, the conveyance direction of the sheet S is reversed by the sheet reversing portion 59, so that the sheet S is conveyed to the image forming portion 55 again.

Next, a configuration of the sheet feeding device 51 according to the present embodiment will be described with reference to FIG. 2. As described above, the sheet feeding device 51 includes the cassette 51 a and the sheet adsorption separation feeding portion 51 b that feeds the sheets S one by one while adsorbing the sheet S stored in the cassette 51 a by static electricity. The sheet feeding device 51 further includes a lifting and lowering unit 301 that is installed to be lifted and lowered in the cassette 51 a and lifts and lowers a sheet supporting plate 301 a in which the sheets S are loaded and the sheet leading end detecting sensor 51 c that detects the passage of the sheet S fed by the sheet adsorption separation feeding portion 51 b.

The sheet adsorption separation feeding portion 51 b includes a pair of nip conveying rollers 501 serving as a nip conveying member configured with an inner nip conveying roller (a first rotating member) 501 a and an outer nip conveying roller 501 (a first nip member) b. The sheet adsorption separation feeding portion 51 b further includes an endless adsorbing member 410 having flexibility nipped between the pair of nip conveying rollers 501, a first driving unit 203, and a power source unit 205. The pair of nip conveying rollers 501 is arranged to be apart from the top surface of the topmost sheet loaded on the cassette 51 a by a predetermined gap Lrgap. The inner surface of the adsorbing member 410 is supported by the inner nip conveying roller 501 a. In the present embodiment, the pair of drawing rollers 51 d and 51 e is arranged above the pair of nip conveying rollers 501. The sheet adsorption separation feeding portion 52 b arranged in the sheet feeding device 52 has the same configuration as the sheet adsorption separation feeding portion 51 b of the sheet feeding device 51, and thus a description thereof is omitted.

The lifting and lowering unit 301 includes a lifter 301 b that is installed to be rotatable down the sheet supporting plate 301 a, and changes the position of the sheet supporting plate 301 a and the position of a topmost sheet Sa loaded on the sheet supporting plate 301 a according to a rotation angle of the lifter 301 b. The sheet leading end detecting sensor 51 c is arranged in the sheet conveyance path between the sheet adsorption separation feeding portion 51 b and the pair of drawing rollers 51 d and 51 e. Success or failure of sheet feeding is detected by detecting whether or not the sheet leading end detecting sensor 51 c detects the sheet S at a predetermined timing. In the present embodiment, the sheet leading end detecting sensor 51 c is a non-contact reflective photo sensor, and detects the presence or absence of a detection target by irradiating the detection target with spotlight and measuring reflected light quantity thereof.

In FIG. 2, 302 indicates a plane of paper height detecting unit that detects the top surface position of the sheet S loaded on the sheet supporting plate 301 a. The plane of paper height detecting unit 302 is arranged above the sheet supporting plate 301 a and configured with a sensor flag 302 a and a photo sensor 302 b. The sensor flag 302 a is rotatably supported on a support portion (not illustrated), and one end of the sensor flag 302 a is arranged at a position at which it can come into contact with the top surface of the topmost sheet Sa, and the other end of the sensor flag 302 a is arranged at a position at which it can light-shield the photo sensor 302 b.

Here, when the top surface of the topmost sheet Sa is positioned at a predetermined height, the sensor flag 302 a rotates, and the photo sensor 302 b is light-shielded. FIG. 3 is a control block diagram of the sheet feeding device 51 according to the present embodiment, and in FIG. 3, 70 indicates a controller. In addition to the sheet leading end detecting sensor 51 c and the plane of paper height detecting unit 302, the first driving unit 203, a positive voltage supply unit 205 a, a negative voltage supply unit 205 b, a timer 71, and the like which will be described later are connected to the controller 70.

Further, the controller 70 detects the position of the top surface of the topmost sheet Sa by detecting the light-shielding state of the photo sensor 302 b. The controller 70 controls an operation of the lifting and lowering unit 301 such that the top surface of the topmost sheet Sa is consistently detected by the plane of paper height detecting unit 302, and maintains the position of the sheet supporting plate 301 a to be a position at which the height of the top surface of the topmost sheet Sa is almost constant. As a result, a gap Lrgap between the driving roller 401 and the top surface of the topmost sheet Sa is maintained to be almost constant.

The inner nip conveying roller 501 a is arranged inside the adsorbing member 410 and rotatably shaft-supported by a shaft support member (not illustrated) whose arrangement position is fixed. Further, driving force is transmitted from the first driving unit 203 to the inner nip conveying roller 501 a through a driving transmission unit (not illustrated). The outer nip conveying roller 501 b is arranged outside the inner nip conveying roller 501 a with the adsorbing member 410 interposed therebetween and rotatably shaft-supported by a shaft support member (not illustrated) to which a pressing spring 501 c is connected. The outer nip conveying roller 501 b is biased in a shaft center direction of the inner nip conveying roller 501 a by the pressing spring 501 c.

The adsorbing member 410 has an endless shape, and includes a positive electrode 410 a and a negative electrode 410 b illustrated in FIG. 4 which will be described later. A high voltage contact point 205 f to which a positive voltage is supplied and a high voltage contact point 205 g to which a negative voltage is supplied are electrically connected to the positive electrode 410 a and the negative electrode 410 b, respectively, as illustrated in FIG. 2. The electrostatic adsorption force of attracting the sheet S is generated in the adsorbing member 410 by the applied voltages supplied from the high voltage contact points 205 f and 205 g. Further, switches 205 c and 205 d for controlling the voltage supply are connected to the positive high voltage contact point 205 f and the negative high voltage contact point 205 g.

Here, in the present embodiment, when the adsorbing member 410 adsorbs and conveys the sheet, the adsorbing member 410 adsorbs the sheet by the static electricity so that the sheets do not undergo sliding friction, and then the adsorbing member 410 is pulled up while being elastically deformed. As the adsorbing member 410 is pulled up while being elastically deformed as described above, the sheet is separated from another sheet. In this regard, in the present embodiment, the length of the adsorbing member 410 is decided so that a sheet contact area Mn (which will be described later) illustrated in (d) of FIG. 6 in which the sheet adsorption force necessary for the adsorption separation is obtained is secured.

In FIG. 2, 206 indicates an adsorbing member position detecting sensor. The adsorbing member position detecting sensor 206 detects a protruding portion 410 g (which will be described later) illustrated in FIG. 5 formed in the adsorbing member 410, and detects a rotation position of the adsorbing member 410 by detecting the protruding portion 410 g. In the present embodiment, the adsorbing member position detecting sensor 206 is a non-contact reflective photo sensor, and detects the presence or absence of a detection target by irradiating the detection target with spot light and measuring reflected light quantity.

204 a and 204 b are a pair of guides that is arranged between the sheet adsorption separation feeding portion 51 b and the pair of drawing rollers 51 d and 51 e and stretch vertically, and guide the sheet S conveyed by the adsorbing member 410 to the pair of drawing rollers 51 d and 51 e thereabove. As illustrated in FIG. 4, the adsorbing member 410 includes a base layer 410 c, the positive electrode 410 a, and the negative electrode 410 b, and the positive electrode of the comb teeth shape and the negative electrode 410 b of the comb teeth shape are alternately arranged inside the base layer 410 c. In the present embodiment, the base layer 410 c is of polyimide serving as a dielectric having volume resistance of 108 Ωcm or more and has a thickness of about 100 μm. The positive electrode 410 a and the negative electrode 410 b are conductors having volume resistance of 106 Ωcm or less and made of copper having a thickness of about 10 μm.

Exposed regions 410 h and 410 i in which the positive electrode and the negative electrode are exposed are formed on the back surface of the adsorbing member 410 as illustrated in FIG. 5. The positive high voltage contact point 205 f illustrated in FIG. 2 which is connected with the positive voltage supply unit comes into contact with the exposed region 410 h of the positive electrode, and the negative high voltage contact point 205 g which is connected with the negative voltage supply unit comes into contact with the exposed region 410 i of the negative electrode.

In the present embodiment, the positive high voltage contact point 205 f and the negative high voltage contact point 205 g have a structure in which a carbon brush is caulked to a leading end of a metallic plate having elasticity, and the carbon brush comes into contact with the exposed regions 410 h and 410 i. Since the positive high voltage contact point 205 f and the negative high voltage contact point 205 g have the elasticity, it is possible to stably supply electric power while following the adsorbing member 410 whose cross-sectional shape changes from hour to hour.

In the adsorbing member 410, a straight line portion 410 m and a curved portion 410 n are formed along the circumferential direction as illustrated in FIG. 5. A reinforcing member 410 p is coupled to only the straight line portion 410 m of the base layer 410 c configuring the entire circumferential direction region of the adsorbing member 410, and a protruding portion 410 g is formed on a part of an end surface of the curved portion 410 n of the adsorbing member 410.

Next, a detailed configuration of the adsorbing member 410 and a generation principle of adsorption force by which the adsorbing member 410 adsorbs the sheet S will be described with reference to FIG. 4. (a) of FIG. 4 is a diagram illustrating the surface of the adsorbing member, (b) of FIG. 4 is a perspective view of the adsorbing member 410, (c) of FIG. 4 is a diagram illustrating a cross section of the power supply portion of the adsorbing member 410, and (d) of FIG. 4 is a diagram illustrating a concept of the electrostatic adsorption force working between the adsorbing member 410 and the sheet S.

Further, in the present embodiment, as will be described later, when the adsorbing member 410 approaches the sheet S, the adsorbing member 410 has appropriate elasticity by adjusting, for example, a material and a thickness of the adsorbing member 410 so that the adsorbing member 410 is bent downward to become a barrel shape. Exposed regions 410 d and 410 e serving as a conducting portion in which the positive electrode 410 a and the negative electrode 410 b are exposed are formed on the inner circumferential surface of the adsorbing member 410 that approaches the inner nip conveying roller 501 a and the outer nip conveying roller 501 b. The high voltage contact point 205 f connected with the positive voltage supply unit 205 a that applies the positive voltage to the positive electrode 410 a via a switch 205 c illustrated in FIG. 2 comes into contact with the exposed region 410 d of the positive electrode 410 a serving as one electrode. The high voltage contact point 205 g connected with the negative voltage supply unit 205 b which is a second power source that applies the negative voltage to the negative electrode 410 b via a switch 205 d illustrated in FIG. 2 comes into contact with the exposed region 410 e of the negative electrode 410 b serving as the other electrode.

Further, in the present embodiment, the positive voltage of about +1 kV is applied to the positive electrode 410 a, and the negative voltage of about −1 kV is applied to the negative electrode 410 b. The high voltage contact point 205 f and the high voltage contact point 205 g have a structure in which a carbon brush is caulked to a leading end of a metallic plate having elasticity, and the carbon brush comes into contact with the exposed regions 410 d and 410 e of the positive electrode 410 a and the negative electrode 410 b. Since the high voltage contact point 205 f and the high voltage contact point 205 g have the elasticity, the high voltage contact point 205 f and the high voltage contact point 205 g can come into contact with the adsorbing member 410 while following the adsorbing member 410 whose cross-sectional shape changes from hour to hour, and thus electric power can be stably supplied.

Here, as illustrated in (d) of FIG. 4, when the positive and negative voltages are applied to the positive electrode 410 a and the negative electrode 410 b, respectively, an unequal electric field is formed near the surface of the adsorbing member 410 due to the positive electrode 410 a and the negative electrode 410 b to which the voltages are applied. When the adsorbing member 410 in which the unequal electric field is formed approaches the sheet S, dielectric polarization occurs on the surface layer of the sheet serving as a dielectric, and electrostatic adsorption force is generated between the adsorbing member 410 and the sheet S due to Maxwell's stress.

Next, the sheet feeding operation of the sheet adsorption separation feeding portion 51 b according to the present embodiment will be described. (a) of FIG. 6 illustrates an initial operation of arranging the adsorbing member 410 at the initial feed operation position. Through this operation, the adsorbing member 410 is positioned at the position at which the protruding portion 410 g is detected by the adsorbing member position detecting sensor 206. Further, through this operation, the curved portion 410 n of the adsorbing member 410 is nipped between the pair of nip conveying rollers 501, and the adsorbing member 410 is separated from the topmost sheet Sa by a predetermined gap Lbgap. At this time, the first driving unit 203 is stopped, and thus the switches 205 c and 205 d are in a non-conduction state.

(b) of FIG. 6 illustrates an approach operation of operating the first driving unit 203 and conveying the adsorbing member 410 to the sheet conveyance downstream through the pair of nip conveying rollers 501 to approach the sheet. At the time of this operation, the controller 70 activates the first driving unit 203, and causes the pair of nip conveying rollers 501 to rotate in the arrow F direction. Then, when the pair of nip conveying rollers 501 is rotated as described above, the curved portion 410 n of the adsorbing member 410 approaches the topmost sheet Sa.

(c) of FIG. 6 illustrates a contact area increase operation of causing the surface of the adsorbing member 410 to come into surface contact with the loaded topmost sheet Sa by performing the approach operation continuously and thus increasing the contact area Mc. At the time of this operation, similarly to the approach operation, the controller 70 increases the contact area Mc by causing the pair of nip conveying rollers 501 to rotate in the arrow F direction and conveying the adsorbing member 410 in the arrow F direction. Then, the contact area increase operation is continuously performed until the contact area Mc is equal to a predetermined contact area Mn illustrated in (d) of FIG. 6.

(d) of FIG. 6 illustrates an adsorption operation of nipping the straight line portion 410 m of the adsorbing member 410 through the pair of nip conveying rollers 501 and causing the adsorbing member 410 to adsorb the topmost sheet Sa in a state in which the surface of the adsorbing member 410 comes into surface contact with the topmost sheet Sa by the predetermined contact area Mn. At the time of this operation, the controller 70 causes the switches 205 c and 205 d to enter a conduction state. As a result, the voltages are applied to the adsorbing member 410 through the high voltage contact points 205 f and 205 g, and the electrostatic adsorption force works between the adsorbing member 410 and the sheet Sa. Further, in the present embodiment, when the contact area becomes Mn, the first driving unit 203 is stopped during a predetermined period of time.

(e) of FIG. 6 illustrates a separation operation of nipping and conveying the straight line portion 410 m of the adsorbing member 410 through the pair of nip conveying rollers 501 and separating the sheet Sa adsorbed on the adsorbing member 410 from the loaded next sheet Sb. At the time of this operation, the controller 70 operates the first driving unit 203, and pulls up the adsorbing member 410 adsorbing the topmost sheet Sa through the pair of nip conveying rollers 501. At this time, since the pair of nip conveying rollers 501 nips the straight line portion 410 m of the adsorbing member 410, the sheet Sa adsorbed on the adsorbing member 410 is linearly pulled up as indicated by an arrow A and separated from the loaded next sheet Sb.

(f) of FIG. 6 illustrates a conveyance operation of conveying the adsorbing member 410 through the operation of the first driving unit 203 after the separation operation and adsorbing and conveying the topmost sheet Sa up to the pair of drawing rollers 51 d and 51 e. Here, when the leading end of the topmost sheet Sa is conveyed to the downstream further than a nip portion formed by the pair of nip conveying rollers 501, the leading end of the topmost sheet Sa is peeled off from the adsorbing member 410 through the pair of guides 204 a. Thereafter, the peeling of the topmost sheet Sa is increased from the leading end, but the topmost sheet Sa is conveyed by adsorption of the rear end region without change and handed over to the pair of drawing rollers 51 d and 51 e through detection of the leading end by the sheet leading end detecting sensor 51 c.

The conveyance operation is continuously performed until the rear end of the topmost sheet Sa is detected by the sheet leading end detecting sensor 51 c and stopped when the adsorbing member 410 returns to the initial position illustrated in (a) of FIG. 6. Further, when the sheet Sa has not been detected by the sheet leading end detecting sensor 51 c, it is determined that there is a mistake in the feeding operation of the topmost sheet Sa, and the feeding operation is resumed starting from the initial operation. One sheet is fed from a plurality of sheets S loaded on the cassette 51 a through the above six processes. Further, it is possible to continuously feed the sheets S one by one by repeatedly performing the six processes.

FIG. 7 a timing chart of the initial operation, the approach operation, the contact area increase operation, the adsorption operation, the separation operation, and the conveyance operation illustrated in FIG. 6. Here, a configuration in which the conveying is performed under the assumption that the long side of the sheet S of the A4 size is the leading end, and the perimeter of the adsorbing member 410 is 100 mm is illustrated as an example. In other words, in the present embodiment, since the length of the sheet S in the sheet feeding direction is 210 mm, it is possible to feed the sheet S by rotation of slightly over two rotations of the adsorbing member 410.

In FIG. 7, a zone from a time T0 to a time T1 indicated by (a) is an initial operation zone, and at this time, a conveyance velocity u is set to 0, and a supply voltage vp and a supply voltage vn are set to 0 (the non-conduction state). The adsorbing member position detecting sensor 206 is in a detected state, and the sheet leading end detecting sensor 51 c is in a non-detected state. A zone from the time T1 to a time T3 indicated by (b) is an approach operation zone, and the conveyance velocity is set to U at the time T1, and the adsorbing member position detecting sensor 206 enters the non-detected state at the time T2.

A zone from the time T3 to a time T4 indicated by (c) is a contact area increase operation zone, and subsequently to the time T1, the conveyance velocity u is set to the velocity U. A zone from the time T4 to a time T5 indicated by (d) is an adsorption operation zone, and at the time T4, the conveyance velocity u is set to 0, the supply voltage vp and the supply voltage vn are set to +V and −V, respectively. A zone from the time T5 to a time T6 indicated by (e) is a separation operation zone, and the conveyance velocity u is set to U at the time T5, and the supply voltage vp and the supply voltage vn are set to +V and −V, respectively, continuously from the time T4.

A zone from the time T6 to a time T10 indicated by (f) is a conveyance operation zone, and the conveyance velocity u is set to U, and the supply voltage vp and the supply voltage vn are set to +V and −V, respectively. At the time T7, the sheet leading end detecting sensor 51 c enters the detected state, and since the adsorbing member 410 can convey the sheet S by slightly over two rotations, after the adsorbing member position detecting sensor 206 enters the detected state twice, the sheet leading end detecting sensor 51 c enters the non-detected state at the time T8.

Then, when the adsorbing member position detecting sensor 206 enters the detected state at the time T9, all of the conveyance velocity u, the supply voltage vp, and the supply voltage vn are set to 0 at the time T10. The controller 70 determines whether or not the feeding is retried according to whether or not the time T7 falls within a predetermined value range. A zone from the time T10 to a time T11 indicated by (a) is the initial operation, and preparation for feeding of the next sheet S is performed. Thereafter, the above operation is repeated, and thus continuous sheet feeding is performed.

Further, in the present embodiment, the straight line portion 410 m of the adsorbing member 410 is formed by adding the reinforcing member 410 p to the base layer 410 c, but even when the base layer 410 c is molded in the shape of the straight line portion 410 m and the curved portion 410 n, the same effects can be obtained. The straight line portion 410 m and the curved portion 410 n may be formed of materials that differ in hardness.

Further, in the present embodiment, the electrostatic adsorption force is generated between the adsorbing member 410 and the sheet S through the above configuration, but the present embodiment is not limited to this example. For example, the positive electrode 410 a and the negative electrode 410 b may not have the comb teeth shape and may have a shape of a uniform electrode in which the electric field can be formed between the electrodes 410 a and 410 b and the sheet S to dielectric-polarize the sheet S.

The present embodiment has been described in connection with the example in which the adsorbing member 410 includes the straight line portion 410 m and the curved portion 410 n, but the present invention is not limited to this example. In other words, the present invention may be a configuration in which the adsorbing member 410 includes a first portion and a second portion having a larger curvature than the first portion. Thus, it is possible to adsorb the sheet through the second portion and conveying the sheet upward through the first portion. In order to convey the sheet upward, it is desirable that the first portion have substantially a straight line form.

As described above, in the present embodiment, the adsorbing member 410 is movable to the adsorption position at which the adsorbing member 410 comes into surface contact with the sheet and adsorbs the sheet, the separation position at which the adsorbed sheet is separated from the lower sheet while eliminating the bending, and the separation position at which the adsorbed sheet is separated. Further, the adsorbing member 410 rotates and adsorbs the sheet and hands the adsorbed sheet over to the pair of drawing rollers 51 d and 51 e, and thereafter, the adsorbing member 410 is stopped at a position (a standby position) away from the sheet.

Further, in the present embodiment, the adsorbing member 410 is configured with the straight line portion 410 m and the curved portion 410 n. The adsorbing member 410 is moved to the adsorption position at which the curved portion 410 n comes into surface contact with the sheet by the rotation of the pair of nip conveying rollers 501, and thereafter, the adsorbing member 410 is pulled up toward the pair of drawing rollers 51 d and 51 e while eliminating the bending by the straight line portion 410 m. As a result, even when the pair of drawing rollers 51 d and 51 e is arranged above the adsorbing member 410, it is possible to hand the adsorbed sheet over to the pair of drawing rollers 51 d and 51 e arranged above the adsorbing member 410 when the adsorbing member 410 is rotated. As a result, it is possible to stably perform sheet feeding by the electrostatic adsorption at a low noise with a simple configuration. Further, it is possible to reduce the length of the sheet feeding device in the sheet feeding direction.

Next, a second embodiment of the present invention will be described. FIG. 8 is a diagram for describing a configuration of a sheet feeding device according to the present embodiment. In FIG. 8, the same reference numerals as those in FIG. 2 denote the same or corresponding parts.

In FIG. 8, 461 indicates a pair of first nip conveying rollers, 462 indicates a pair of second nip conveying rollers, and 460 indicates an endless adsorbing member having flexibility which is nipped and conveyed by the pair of first nip conveying rollers 461 and the pair of second nip conveying rollers 462. The pair of first nip conveying rollers 461 and the pair of second nip conveying rollers 462 serving as a plurality of nip conveying members arranged in the sheet feeding direction are arranged to be apart from the top surface of the topmost sheet Sa loaded on the cassette 51 a by a predetermined gap Lrgap. The pair of second nip conveying rollers 462 serving as a second nip conveying member is arranged in the upstream further than the pair of first nip conveying rollers 461 serving as the first nip conveying member in the sheet feeding direction.

The pair of first nip conveying rollers 461 includes a first inner nip conveying roller (a first rotating member) 461 a and a first outer nip conveying roller (a first nip member) 461 b, and the pair of second nip conveying rollers 462 includes a second inner nip conveying roller (a second rotating member) 462 a and a second outer nip conveying roller (a second nip member) 462 b. The first inner nip conveying roller 461 a is arranged inside the adsorbing member 460 and rotatably shaft-supported by a shaft support member (not illustrated) whose arrangement position is fixed. Driving force is transmitted from the first driving unit 203 to the first inner nip conveying roller 461 a through a driving transmission unit (not illustrated).

The first outer nip conveying roller 461 b is arranged outside the first inner nip conveying roller 461 a with the adsorbing member 460 interposed therebetween and rotatably shaft-supported by a shaft support member (not illustrated) to which a first pressing spring 461 c is connected. The first outer nip conveying roller 461 b is biased in a shaft center direction of the first inner nip conveying roller 461 a by the first pressing spring 461 c.

Similarly to the first inner nip conveying roller 461 a, the second inner nip conveying roller 462 a is arranged inside the adsorbing member 460 and rotatably shaft-supported by a shaft support member (not illustrated) whose arrangement position is fixed. Driving force is transmitted from the second driving unit 204 to the second inner nip conveying roller 462 a through a driving transmission unit (not illustrated).

Similarly to the first outer nip conveying roller 461 b, the second outer nip conveying roller 462 b is arranged outside the second inner nip conveying roller 462 a with the adsorbing member 460 interposed therebeteween and rotatably shaft-supported by a shaft support member (not illustrated) to which a second pressing spring 462 c is connected. The second outer nip conveying roller 462 b is biased in a shaft center direction of the second inner nip conveying roller 462 a by the second pressing spring 462 c.

The adsorbing member 460 is wound on the pair of first nip conveying rollers 461 and the pair of second nip conveying rollers 462, and has a length larger than the shortest length that can be wound on the pair of first nip conveying rollers 461 and the pair of second nip conveying rollers 462. Since the adsorbing member 460 has this length, there is a gap between the pair of first nip conveying rollers 461 and the pair of second nip conveying rollers 462 and the top surface of the topmost sheet, but the adsorbing member 460 can come into contact with the top surface of the topmost sheet Sa.

The positive voltage supply unit 205 a and the negative voltage supply unit 205 b are electrically connected to the adsorbing member 460, and the electrostatic adsorption force of attracting the sheet S is generated in the adsorbing member 460 by the voltages applied from the positive and negative voltage supply units 205 a and 205 b. The switches 205 c and 205 d that control the voltage supply are connected to the positive high voltage contact point 205 f and the negative high voltage contact point 205 g, respectively.

In FIG. 8, 463 indicates a first adsorbing member position detecting sensor, 464 indicates a second adsorbing member position detecting sensor, and the first and second adsorbing member position detecting sensors 463 and 464 detect protruding portions 460 j and 460 k of the adsorbing member 460 illustrated in FIG. 9 which will be described later. The first and second adsorbing member position detecting sensors 463 and 464 detect a rotation position of the adsorbing member 460 by detecting the protruding portions 460 j and 460 k of the adsorbing member 460, respectively. Further, in the present embodiment, the first and second adsorbing member position detecting sensors 463 and 464 are non-contact reflective photo sensors, and detect the presence or absence of a detection target by irradiating the detection target with spot light and measuring reflected light quantity.

In the adsorbing member 460, as illustrated in FIG. 9, straight line portions 460 a and 460 b and curved portions 460 c and 460 d are formed in the circumferential direction to face each other. Reinforcing members 460 f and 460 g are coupled with only the straight line portions 460 a and 460 b of a base layer (a base member) 460 e configuring an entire circumferential direction region of the adsorbing member 460.

Further, exposed regions 460 h and 460 i in which the positive electrode and the negative electrode are exposed are formed on the inner side of the adsorbing member 460, and the protruding portions 460 j and 460 k are formed on a part of an end surface of the adsorbing member 460. Here, the positive high voltage contact point 205 f connected with the positive voltage supply unit illustrated in FIG. 8 comes into contact with the exposed region 460 h of the positive electrode, and the negative high voltage contact point 205 g connected with the negative voltage supply unit comes into contact with the exposed region 460 i of the negative electrode. Further, in the present embodiment, the positive and negative voltages of about 1 kV are applied to the positive electrode and the negative electrode, respectively.

Next, the sheet feeding operation of the sheet adsorption separation feeding portion 51 b according to the present embodiment will be described. (a) of FIG. 10 illustrates an initial operation of arranging the adsorbing member 460 at the initial feed operation position. Through this operation, the adsorbing member 460 is positioned at the position at which the protruding portions 410 j and 460 k are detected by the first and second adsorbing member position detecting sensors 463 and 464, respectively. Further, through this operation, the curved portions 460 c and 460 d are nipped by the pair of first nip conveying rollers 461 and the pair of second nip conveying rollers 462, respectively, and the adsorbing member 460 is separated from the topmost sheet Sa by a predetermined gap Lbgap. At this time, the first driving unit 203 and the second driving unit 204 are stopped, and thus the switches 205 c and 205 d are in the non-conduction state.

(b) of FIG. 10 illustrates an approach operation of operating the first driving unit 203 and the second driving unit 204 and conveying the adsorbing member 460 to the downstream in the sheet conveyance direction through the pair of first nip conveying rollers 461 and the pair of second nip conveying rollers 462 to approach the sheet. At the time of this operation, the controller 70 activates the first driving unit 203 and the second driving unit 204, and rotates the pair of first nip conveying rollers 461 and the pair of second nip conveying rollers 462 in the arrow F direction. Then, when the pair of first nip conveying rollers 461 and the pair of second nip conveying rollers 462 are rotated as described above, the curved portion 460 d (or 460 c) of the adsorbing member 460 approaches the topmost sheet Sa.

(c) of FIG. 10 illustrates a contact area increase operation of causing the surface of the adsorbing member 460 to come into surface contact with the loaded topmost sheet Sa by performing the approach operation continuously and thus increasing the contact area Mc. At the time of this operation, similarly to the approach operation, the controller 70 continuously conveys the adsorbing member 460 in the arrow F direction until the contact area Mc is equal to the predetermined contact area Mn. Further, in the present embodiment, the first driving unit 203 is stopped in a state in which the straight line portion 460 a (or 460 b) of the adsorbing member 460 is nipped between the pair of first nip conveying rollers 461, and the curved portion 460 d (or 460 c) of the adsorbing member 460 comes into surface contact with the sheet Sa.

Then, only the second driving unit 204 is continuously driven to increase the contact area Mc. Further, the switches 205 c and 205 d enter the condition state, and thus since the voltages are applied through the positive voltage supply unit 205 a and the negative voltage supply unit 205 b of the adsorbing member 460, the electrostatic adsorption force works between the adsorbing member 460 and the topmost sheet Sa.

(d) of FIG. 10 illustrates an adsorption operation of nipping the straight line portion 460 a (or 460 b) of the adsorbing member 460 through the pair of first and second nip conveying rollers 461 and 462 and causing the adsorbing member 460 to adsorb the sheet Sa in a state in which the surface of the adsorbing member 460 comes into surface contact with the sheet Sa by the predetermined contact area Mn. At the time of this operation, the controller 70 stops the first driving unit 203 and the second driving unit 204 during a predetermined period of time in the state in which the surface of the adsorbing member 460 comes into surface contact with the sheet Sa by the predetermined contact area Mn.

(e) of FIG. 10 illustrates a separation operation of conveying the adsorbing member 460 adsorbing the topmost sheet Sa through the pair of first nip conveying rollers 461 and the pair of second nip conveying rollers 462 and separating the topmost sheet Sa from the loaded next sheet Sb. At the time of this operation, the controller 70 first operates the first driving unit 203 after the adsorption operation, and then operates the second driving unit 204 after a predetermined period of time.

As a result, since the pair of first nip conveying rollers 461 nips and conveys the straight line portion 460 a of the adsorbing member 460, the topmost sheet Sa adsorbed on the adsorbing member 460 is linearly pulled up as indicated by the arrow A and separated from the loaded next sheet Sb. Further, in the present embodiment, the two straight line portions 460 a and 460 b are arranged to face each other in the circumferential direction. Thus, when the straight line portion 460 a of the adsorbing member 460 is nipped between the pair of first nip conveying rollers 461, the other straight line portion 460 b is also nipped between the pair of second nip conveying rollers 462. Accordingly, it is possible to pull up the adsorbing member 460 with certainty.

(f) of FIG. 10 illustrates a conveyance operation of conveying the adsorbing member 460 through the operations of the first driving unit 203 and the second driving unit 204 after the separation operation and adsorbing and conveying the topmost sheet Sa up to the pair of drawing rollers 51 d and 51 e. At this time, the topmost sheet Sa is adsorbed on the adsorbing member 460, and the topmost sheet Sa is conveyed together with the adsorbing member 460 while maintaining the state in which the topmost sheet Sa is separated from the loaded next sheet Sb.

Further, when the leading end of the topmost sheet Sa is conveyed to the downstream further than the nip portion formed by the pair of first nip conveying rollers 461, the leading end of the topmost sheet Sa is peeled off from the adsorbing member 460 through the pair of guides 204 a. Thereafter, the peeling of the topmost sheet Sa is increased from the leading end, but the topmost sheet Sa is conveyed by adsorption of the rear end region without change and handed over to the pair of drawing rollers 51 d and 51 e through detection of the leading end by the sheet leading end detecting sensor 51 c. The conveyance operation is continuously performed until the rear end of the topmost sheet Sa is detected by the sheet leading end detecting sensor 51 c and stopped when the adsorbing member 460 returns to the initial position illustrated in (a) of FIG. 10. Further, when the sheet Sa has not been detected by the sheet leading end detecting sensor 51 c, it is determined that there is a mistake in the feeding operation of the topmost sheet Sa, and the feeding operation is resumed starting from the initial operation. One sheet is fed from a plurality of sheets S loaded on the cassette 51 a through the above six processes. Further, it is possible to continuously feed the sheets S one by one by repeatedly performing the six processes.

FIG. 11 is a timing chart of the feeding operation of feeding one sheet through the sheet adsorption separation feeding portion 51 b described with reference to FIG. 10. In the present embodiment, a configuration in which the conveying is performed under the assumption that the long side of the sheet S of the A4 size is the leading end, and the perimeter of the adsorbing member 460 is 410 mm is illustrated as an example. In other words, in the present embodiment, since the length of the sheet S in the sheet feeding direction is 210 mm, it is possible to feed the sheet S by rotation of slightly over one rotation of the adsorbing member 410.

In FIG. 11, a zone from a time T0 to a time T1 indicated by (a) is an initial operation zone, and the first and second adsorbing member position detecting sensors 463 and 464 are in the detected state, and the sheet leading end detecting sensor 51 c is in the non-detected state. The conveyance velocity u1 and the conveyance velocity u2 are set to 0 (the stop state), and the supply voltage vp and the supply voltage vn are set to (the non-conduction state). A zone from the time T1 to a time T3 indicated by (b) is an approach operation, and the conveyance velocity u1 and the conveyance velocity u2 are set to U at the time T1, and the first and second adsorbing member position detecting sensors 463 and 464 enters the non-detected state at the time T2.

In FIG. 11, a zone from the time T3 to a time T5 indicated by (c) is a contact area increase operation. The conveyance velocity u1 and the conveyance velocity u2 are set to the velocity U continuously from the time T1, but at the time T4, only the conveyance velocity u1 is set to 0, the supply voltage vp is set to +V, and the supply voltage vn is set to −V. A zone from the time T5 to a time T6 indicated by (d) is an adsorption operation, and the conveyance velocity u2 is set to 0 at the time T5. A zone from the time T6 to a time T7 indicated by (e) is a separation operation, and the conveyance velocity u1 is set to the velocity U at the time T6.

In FIG. 11, a zone from the time T7 to a time T11 indicated by (f) is a conveyance operation, and the conveyance velocity u2 is set to the velocity U at the time T7. At the time T8, the sheet leading end detecting sensor 51 c enters the detected state. Then, since the adsorbing member 460 can convey the topmost sheet Sa by slightly over one rotation, the sheet leading end detecting sensor 51 c enters the non-detected state at the time T9 after the first and second adsorbing member position detecting sensors 463 and 464 enter the detected state twice.

Then, when the first and second adsorbing member position detecting sensors 463 and 464 enter the detected state at the time T10, at the time T11, all of the conveyance velocity u1, the conveyance velocity u2, the supply voltage vp, and the supply voltage vn are set to 0. The controller 70 determines whether or not the feeding is retried according to whether or not the time T8 falls within a predetermined value range. A zone from the time T11 to a time T12 indicated by (a) is the initial operation, and preparation for feeding of the next sheet S is performed. Thereafter, the above operation is repeated, and thus continuous sheet feeding is performed.

As described above, in the present embodiment, the adsorbing member 460 is nipped and conveyed by the pair of first nip conveying rollers 461 and the pair of second nip conveying rollers 462. As described above, by employing the configuration, it is possible to increase the adsorption area between the adsorbing member 460 and the sheet S, and it is possible to obtain solid conveyance force. Further, in the present embodiment, the reinforcing members 460 f and 460 g are coupled with only the straight line portions 460 a and 460 b of the base layer (base member) 460 e, and thus the magnitude of the stiffness of the straight line portions 460 a and 460 b is larger than the magnitude of the stiffness of the curved portions 460 d and 460 c. However, the present embodiment is not limited to this example, and the straight line portions 460 a and 460 b of the adsorbing member 460 may be configured with any other member having the stiffness larger than that of the curved portions 460 d and 460 c.

Next, a third embodiment of the present invention will be described. FIG. 12 is a diagram for describing a configuration of a sheet feeding device according to the present embodiment. In FIG. 12, the same reference numerals as those in FIG. 8 denote the same or corresponding parts.

In FIG. 12, 470 indicates an adsorbing member, and 250 a indicates a charging roller serving as a voltage applying member that is arranged above the adsorbing member 470 and presses the adsorbing member 470 downward. The charging roller 250 a is rotatably supported by a shaft support member (not illustrated) whose arrangement position is fixed and drivenly rotates with the movement of the adsorbing member 470. The AC power source 252 is connected to the charging roller 250 a. As a result, charges by contact charging by the charging roller 250 a are applied to the surface of the adsorbing member 470, and the electrostatic adsorption force of attracting the sheet S is generated by the applied charges. 250 b is a backup roller that is arranged at a position of the inner circumferential surface of the adsorbing member 470 corresponding to the charging roller 250 a to stably come into contact with the charging roller 250 a and the adsorbing member 470 and presses the adsorbing member 470 upward.

In parallel with the conveyance operation of the adsorbing member 470, an alternating voltage is applied from the charging roller 250 a pressed on the surface of the adsorbing member 470. As a result, a region charged to a positive polarity and a region charged to a negative polarity are formed on the surface of the adsorbing member 470 in a stripe form at intervals corresponding to the frequency of the AC power source 252 and the surface moving velocity of the adsorbing member 470 as illustrated in (a) and (b) of FIG. 13. An unequal electric field is formed near the surface of the adsorbing member 470 by the positive and negative charged regions alternately formed in the stripe form. Further, when the adsorbing member 470 in which the unequal electric field is formed as described above approaches the sheet S, dielectric polarization occurs on the surface layer of the sheet serving as a dielectric, and the electrostatic adsorption force occurs between the adsorbing member 470 and the sheet S by Maxwell's stress.

As described above, in the present embodiment, it is possible to obtain the sheet adsorption force by charging the surface layer of the adsorbing member from the outside by the charging roller 250 a. As a result, since it is possible to charge the adsorbing member 470 without the electrode arranged inside the adsorbing member, it is possible to simplify the configuration of the adsorbing member 470 and reduce the cost.

In the embodiment described so far, the sheet S is adsorbed on the adsorbing member by the electrostatic adsorption force, but the present invention is not limited to this example. For example, a fine fiber structure of a submicron order may be formed on the adsorbing member, and the sheet S may adsorbed by intermolecular attractive force working between the sheet S and the fine fiber structure.

REFERENCE SIGNS LIST

-   51, 52 Sheet feeding device -   51 a Cassette -   51 b, 52 b Sheet adsorption separation feeding portion -   51 c Sheet leading end detecting sensor -   51 d, 51 e Pair of drawing rollers -   55 Image forming portion -   70 Controller -   100 Image forming apparatus -   100A Image forming apparatus body -   203 First driving unit -   204 Second driving unit -   205 Power source unit -   205 a Positive voltage supply unit -   205 b Negative voltage supply unit -   206 Adsorbing member position detecting sensor -   250 a Charging roller -   252 AC power source -   401 Driving roller -   410 Adsorbing member -   410 a Positive electrode -   410 b Negative electrode -   410 m Straight line portion -   410 n Curved portion -   410 g Protruding portion -   410 p Reinforcing member -   460 Adsorbing member -   460 a, 460 b Straight line portion -   460 c, 460 d Curved portion -   460 j, 460 k Protruding portion -   461 Pair of first nip conveying rollers -   462 Pair of second nip conveying rollers -   463 First adsorbing member position detecting sensor -   464 Second adsorbing member position detecting sensor -   470 Adsorbing member -   501 Pair of nip conveying rollers -   501 a Inner nip conveying roller -   501 b Outer nip conveying roller -   Mn Sheet contact area -   S Sheet -   Sa Topmost sheet 

1. A sheet feeding device, comprising: a loading unit that loads a sheet; a first rotating member that is arranged above the loading unit; an adsorbing member that includes a first portion and a second portion having a curvature larger than a curvature of the first portion and electrically adsorbs the sheet loaded on the loading unit, an inner side of the adsorbing member being supported by the first rotating member; a first nip member that nips the adsorbing member together with the first rotating member; a first driving unit that rotates the first rotating member and the first nip member; and a control unit that controls the driving unit, wherein the control unit adsorbs the sheet loaded on the loading unit through the second portion and then conveys the sheet upward through the first portion.
 2. The sheet feeding device according to claim 1, wherein the adsorbing member has flexibility, and is arranged to be movable to a standby position away from the sheet loaded on the loading unit, an adsorption position at which the sheet loaded on the loading unit is adsorbed, a separation position at which the adsorbed sheet moves upwards and is separated from a lower sheet, and a separation position at which the adsorbed sheet is separated from the adsorbing member.
 3. The sheet feeding device according to claim 1, further comprising, a second rotating member that is arranged in an upstream further than the first rotating member in a sheet feed direction; a second nip member that nips the adsorbing member together with the second rotating member; and a second driving unit that rotates the second rotating member and the second nip member, wherein the control unit adsorbs the sheet loaded on the loading unit through the second portion by rotating only the second rotating member and the second nip member, and then conveys the sheet upward through the first portion by rotating only the first rotating member and the first nip member.
 4. The sheet feeding device according to claim 1, wherein the first portion of the adsorbing member is formed by coupling a reinforcing member to a base member.
 5. The sheet feeding device according to claim 1, wherein the first portion has substantially a straight line shape.
 6. The sheet feeding device according to claim 1, further comprising, a power source that applies a voltage to the adsorbing member and provides adsorption force of adsorbing the sheet by static electricity, wherein two electrodes are arranged in the adsorbing member, and the power source includes a first power source that applies a positive voltage to one of the two electrodes and a second power source that applies a negative voltage to the other of the two electrodes.
 7. The sheet feeding device according to claim 3, wherein the two electrodes are arranged in the adsorbing member, the power source includes a first power source that applies a positive voltage to one of the two electrodes and a second power source that applies a negative voltage to the other of the two electrodes, and a conducting portion is formed in the first nip member or the second nip member, one of the first power source and the second power source is connected to one of the two electrodes of the adsorbing member through the conducting portion, and the other of the first power source and the second power source is connected to the other of the two electrodes of the adsorbing member through the conducting portion.
 8. The sheet feeding device according to claim 1, further comprising, a voltage applying member that is arranged between the adsorbing member and the power source, and abuts the adsorbing member before the adsorbing member comes into contact with the sheet, and applies a voltage from the power source to the adsorbing member.
 9. The sheet feeding device according to claim 8, wherein the power source is an alternating current (AC) power source.
 10. The sheet feeding device according to claim 1, wherein a magnitude of adsorption force by static electricity when bending of the adsorbing member is eliminated is set to a magnitude by which the sheet is separated from the adsorbing member due to stiffness of the sheet.
 11. An image forming apparatus, comprising: an image forming portion that forms an image on a sheet; a loading unit that loads a sheet; a first rotating member that is arranged above the loading unit; an adsorbing member that includes a first portion and a second portion having a curvature larger than a curvature of the first portion and electrically adsorbs the sheet loaded on the loading unit, and inner side of the adsorbing member being supported by the first rotating member; a first nip member that nips the adsorbing member together with the first rotating member; a first driving unit that rotates the first rotating member and the first nip member; and a control unit that controls the driving unit, wherein the control unit adsorbs the sheet loaded on the loading unit through the second portion and the conveys the sheet upward through the first portion. 